1. Field of Invention
The present invention relates to an incinerator for burning industrial waste such as dust or sludge discharged from chemical plants and from thermal power plants, and waste such as municipal refuse.
2. Prior Art
Exhaust gas discharged from the boilers which burn fossil fuel contains dust that is collected by a bag-filter, cyclone collector or electric precipitator. However, recently, machines and apparatuses for use in steel industry, chemical industry and electric power industry have become very large, thereby increasing the amount of dust collected per dust collector to a very high extent. For example, the amount of dust collected by dust correctors from exhausted gas from a boiler for the generation of electric power of 1 million Kw output is about 1-3 m.sup.3 per hour, though the amount depends on the amount of dust contained in the exhaust gas, the collecting efficiency of the dust collector used and the bulk density of the dust.
Dust contained in the conbustion gas from the burning of a fossil fuel such as heavy oil has the following characteristics.
(1) The bulk specific gravity is as small as 0.1-0.2 g/cc. (2) The amount of combustible components (carbon and sulphuric acid) is about 90%.
(3) The amount of incombustible components (ash) is about 10%, but composed mostly of elements such as vanadium (V) and nickel (Ni).
The dust described above has a large bulk specific gravity, difficult to handle and transport efficiently, and moreover, creates a public nuisance by scattering dust and acidifying the soil. Therefore, it is difficult to dispose of this type of dust. Consequently, a method has been practised that the dust described above is incinerated to decrease it in volume and elements remaining are concentrated to be recovered.
On the other hand, sludge discharged from thermal power plants is produced during processing of waste water from ejectors for airflow-transporting of dust collected by dust collectors, waste water from coal yards, and the like. The disposal of the sludge described above also causes a public nuisance. Therefore, a method has been practised that sludge after being dehydrated is incinerated in the same manner as the dust.
Heretofore, rotary kilns and fluidized bed furnaces have been mainly used as an incenerator for wastes such as dust and sludge. According to the method using a rotary kiln, an additive functioning as a melting point raiser and a granulation promoter, such as magnesium hydroxide, is applied to the dust or sludge which is then formed into pellets through a pelletizer and thrown into a rotary kiln so as to be incinerated or calcined. However, according to this method, sludge is not fully incinerated. To better incinerate it, it may be required that the dimensions of the rotary kiln should be enlarged and additional devices should be provided for treating harmful gaseous components and malodor. Further, low melting point compounds contained in the waste such as vanadium pentoxide and sodium chloride stick to the inner walls of the kiln. Therefore, attention should be paid to the distribution of temperature in the kiln. On the other hand, according to the method using a fluidized bed furnace, a heat transfer medium such as sand is fluidized by ejecting gas into the furnace to form a fluidized bed in which waste is incinerated. This method has such an advantage that, in general, powdered materials such as dust and sludge are suitable for fluidization and are incinerated easily, and the heat capacity of the furnace is high, so that self-combustion can be readily sustained; whereas, there is such a disadvantage that incombustible components which can be molten, particularly vanadium or sodium compounds are molten in the furnace and stick to the inner wall of the furnace. For example, the dust collected from combustion gases of petroleum fuels contains about 10% ash (incombustible components), which is composed mainly of compounds of vanadium, particularly vanadium pentoxide (V.sub.2 O.sub.5) whose melting point is 670.degree. C. Consequently, when the dust is incinerated in an ordinary fluidizing bed furnace in the self-combustion temperature region ranging from 700.degree. C. to 800.degree. C., vanadium pentoxide (V.sub.2 O.sub.5) is molten and exudes onto the surface of dust particle, whereby the dust particle sticks to the inner wall and the inlet for the gas to the furnace, for example, a perforated plate for introducing gas into the furnace. As a result clinkers are formed in the fluidized bed so that fluidizing of the waste is interrupted and the combustion in a fluidized state is made difficult.
Furthermore, when ammonium (NH.sub.3) gas is used as an electrolyte for accelerating deposition of dust in the electric precipitator or as a reducing agent for eliminating nitrogen oxides (NO.sub.x) in combustion gas of petroleum fuels, the dust collected from the combustion gas contains a large amount of ammonium sulfate [(NH.sub.4).sub.2 SO.sub.4 ]produced in the reaction of NH.sub.3 with SO.sub.3 from the sulfur components contained in the fuel. The ammonium sulfate becomes molten in the furnace in the same manner as the vanadium compounds, sticks to the inner wall of a furnace and the like, thus causing troubles similarly to the above.